Patent GB-1411922-A (GB)
Title: Spectrum analyzer establishing a non linear frequency distribution of power density spectra
IPC: G01R23/16
Filed: 1973-11-05
Abstract: 1411922 Determining power spectral density THOMSON-CSF 5 Nov 1973 [7 Nov 1972] 51336/73 Heading G4A Non-uniformly spaced samples of a power spectral density function at a set of points such as {f i : f i =CR<SP>i</SP>, i=0, 1, ..., n} are obtained from a set of uniformly spaced samples of the function by a process analogous to that of computing a weighted moving average centred at each of the non-uniformly spaced points. The particular arrangement described shows how to produce a constant Q (i.e., constant bandwidth) spectral analysis at sample points f i =CR<SP>i</SP>, from a set of uniformly spaced samples produced, for example, by a fast fourier transform. In essence each of the non-uniformly spaced samples is produced by multiplying the set of uniformly spaced samples by one of the frequency windows shown in Fig. 1, and by summing and normalizing the resulting products, this being accomplished by storing a set of (2M-1) weighting factors corresponding to the ordinates of each frequency window at (2M - 1) points equally spaced around its centre frequency, by taking the uniformly spaced samples that occur within a frequency window sequentially and deciding for each one which of the (2M - 1) weighting factors is nearest to it (on the frequency axis), and by forming ans summing the respective products. In more detail, the (2M - 1) weighting factors W j , j=1,2, ..., 2M - 1, for frequency window i centred at CR<SP>i</SP>, and having boundaries (Ck)R<SP>i</SP>, are the ordinates of the window at frequencies f j  = (C - k)R<SP>i</SP> + j k R<SP>i</SP>/M, j=1,2, ... 2M - 1, where 2k is a constant equal to the width of the window at i = 0. The uniformly spaced samples are assumed to exist for unity increments of frequency, so that the frequency u 1  at which the first of the uniformly spaced samples within the window i occurs is where the square brackets [ ] denote "the integral part of". Hence the distance x 1  along the frequency axis from the lower boundary (C-k)R<SP>i</SP> of the window to the first uniform sample frequency u 1  is and the distance to the jth uniform sample frequency u j  is Now each frequency window i is divided by the ordinates of the (2M - 1) weighting factors W l  into 2M equal portions each of width kR<SP>i</SP>/M=S i  (say), so that the ordinal number l j  of the weighting factor Wl j  that is nearest, on the frequency axis, to the jth uniform sample frequency u j  is the integer nearest to x j /S i , i.e. the ordinal number l j  is given by  Thus the general step for the calculation of the non-uniform sample for the ith frequency window is: (a) form x j =x j-1 +1; (b) form l j ; (c) read out from store the uniform sample for frequency u j  and the weighting factor of ordinal number l j ; (d) form and accumulate the product of the uniform sample and the weighting factor, and when every uniform sample within the frequency window has been processed, which can be detected by noting when l j  exceeds (2M-1), normalize the resulting sum of products by dividing by the sum of the weighting factors used. Two block diagrams (Figs. 4 and 5, not shown) are described for performing the necessary calculations.